An example of a conventional time-stamping protocol is given by the IEEE 1588-2008 standard. IEEE 1588-2008 defines the time at which a packet leaves or arrives as the instant the first bit of the start of packet (SOP) leaves or arrives at the input/output pin. This definition is useful, for example, when data is either carried on a single pin pair, as in the Serial Gigabit Media Independent Interface (SGMII), or over a bus where the bus arrives synchronously and the SOP is indicated by bit[0] of the bus, as in the Gigabit Media Independent Interface (GMII).